Mental Time Travel: Can Animals Recall the Past and Plan for the Future? N. S. Clayton and A. Dickinson, University of Cambridge, Cambridge, UK ã 2010 Elsevier Ltd. All rights reserved. Introduction most of us know when and where we were born, we do not remember the birth itself nor the episode in which we were In an influential paper that was published in 1997, Sud- told when our birthday is, and therefore such memories are dendorf and Corballis argued that we humans are unique classified as semantic as opposed to episodic. among the animal kingdom in being able to mentally Episodic and semantic memory, then, are thought to be dissociate ourselves from the present. To do so, we travel marked by two separate states of awareness; episodic backwards and forwards in the mind’s eye to remember and remembering requires an awareness of reliving the past reexperience specific events that happened in the past events in the mind’s eye and of mentally traveling back in (episodic memory) and to anticipate and preexperience one’s own mind’s eye to do so, whereas semantic knowing future scenarios (future planning). Although physical time only involves an awareness of the acquired information travel remains a fictional conception, mental time travel is without any need to travel mentally back in time to something we do for a living, and the fact that we spend so personally reexperience the past event. It is for this reason much of our time thinking about the past and the future that in later writings, Tulving has argued that one of the led to Mark Twain’s witty remark that ‘‘my life has been cardinal features of episodic memory is that it operates in filled with many tragedies, most of which never occurred.’’ ‘subjective time,’ and he refers to the awareness of such Mental time travel then has two components: a retro- subjective time as chronesthesia. spective one in the form of episodic memory and a Language-based reports of episodic recall suggest that prospective one in the form of future planning. In formu- the retrieved experiences are not only explicitly located in lating their mental time travel hypothesis, Suddendorf and the past but are also accompanied by the conscious expe- Corballis were the first to suggest that episodic memory rience of one’s recollections, feeling that one is the author and future planning are intimately linked and can be viewed of the memory, or of traveling back not in any mind’s eye as two sides of the same coin so to speak. In fact, their but in my mind’s eye, what Tulving called autonoetic con- proposal consisted of two claims. In addition to integrating sciousness. In other words, Tulving and others argue that the retrospective and prospective components of mental episodic memory differs from semantic memory not only time travel, they also argued that such abilities were unique in being oriented to the past, but specifically in the past of to humans and reflected a striking cognitive dichotomy the owner of that memory. So while some semantic knowl- between ourselves and other animals. The latter idea was edge, such as the birth date example described earlier, does not new, however, but rather an extension of what others involve a datable occurrence, these memories are funda- have argued makes episodic memory special. mentally distinct from episodic memories because they do Indeed in his seminal studies of human memory, not require any mental time travel. As William James so Tulving coined the term episodic memory in 1972 to aptly wrote ‘‘Memory requires more than the mere dating refer to the recollection of specific personal happenings, of a fact in the past. It must be dated in my past.’’ a form of memory that he claimed was uniquely human From a biological perspective, the characterization of and fundamentally distinct from semantic memory, the episodic memory in terms of these two phenomenologi- ability to acquire general factual knowledge about the cal properties of consciousness, namely autonoesis and world, which he argued we share with most, if not all, chronesthesia, presents major problems for two reasons. animals. Ever since he made this remember–know distinc- The first is that positing a subjective state of awareness is tion, most cognitive psychologists and neuroscientists have difficult to integrate with evolutionary processes of natu- assumed that episodic memory is special because of the ral and sexual selection, which operates on behavioral experiential nature of these memories, namely that our attributes such as reproductive success and survival rather episodic reminiscences are accompanied by a subjective than on mental states. The second is that this definition awareness of currently reexperiencing an event that hap- makes it impossible to test in nonverbal animals, in the pened in the past, as opposed to just knowing that it hap- absence any agreed behavioral markers of non-Linguistic pened. Of course we also have many instances of knowledge consciousness. Adopting an ethological approach to com- acquisition in which we do not remember the episode in parative cognition necessitates two requirements. The which we acquired that information. For example, although first is that the memory needs to be characterized in 438 Mental Time Travel: Can Animals Recall the Past and Plan for the Future? 439 terms of behaviorally defined properties as opposed to it seems likely that the selection pressure for an excellent purely phenomenological ones, such as the types of infor- memory for the caches would have been particularly mation encoded. Indeed, we shall argue that the ability to strong, especially as they cache year round. remember what happened, where and how long ago is a These birds also cache reliably in the laboratory, critical behavioral criterion for episodic memory. The providing both ethological validity and experimental second requirement is the identification of an ethological control. At issue, however, is whether or not these birds context in which these memories would confer a selective episodically remember the past and plan for the future. advantage. Note that by doing so, we transform this debate For these reasons, we shall now turn our attention to about the human uniqueness of mental time travel into an assessing the evidence as to whether or not these food- empirical evaluation in non-Linguistic animals as caching corvids can remember the past and plan for opposed to restricting it to the realms of philosophical the future. personal ponderings. But before doing so, let us return to the two claims made by the mental time travel hypothesis: (1) future planning and episodic memory are subserved by Episodic Memory a common process, mental time travel, and (2) this process As we stated earlier, language-based reports of episodic is uniquely human. We shall evaluate each of these claims recall in humans suggest that the retrieved experiences are in turn, and argue that there is good evidence to support not only explicitly located in the past but are also accom- the first claim, but that considerably more controversy panied by the conscious experience of one’s recollections. surrounds the second component of Suddendorf and Cor- From a comparative perspective, the problem with this ballis’ thesis. definition, however, is that in the absence of agreed non- Evidence to support the first claim comes from a num- Linguistic markers of consciousness, it is not clear how one ber of sources. First, studies of brain activity while engaged could ever test whether animals are capable of episodic in either memory retrieval or future-oriented tasks iden- recollection. For how would one assess whether or not an tify a specific core network of regions in the brain of animal can experience an awareness of the passing of time healthy human adults that support both episodic recollec- and of reexperiencing one’s own memories while retriev- tion and future planning. Moreover, there are patients such ing information about a specific past event. as DB and KC, who show specific impairments in episodic but not semantic memory, and these patients have similar Behavioral criteria for episodic memory deficits in episodic but not semantic forethought. Finally, This dilemma can be resolved to some degree, however, studies of cognitive development in young children sug- by using Tulving’s original definition of episodic memory, gest that episodic memory and future planning both in which he identified episodic recall as the retrieval of emerge at about the same age, and are not properly devel- information about ‘where’ a unique event occurred, ‘what’ oped until children reach the age of about four. happened during the episode, and ‘when’ it took place. The advantage of using this definition is that the simul- Is Mental Time Travel Unique to Humans? taneous retrieval and integration of information about these three features of a single, unique experience may be demonstrated behaviorally in animals. Clayton and Regarding the second claim about the uniqueness of epi- Dickinson termed this ability ‘episodic-like memory’ rather sodic memory and future planning, if we are to adopt an than episodic memory because we have no way of know- ethological approach of the form we outlined earlier, then ing whether or not this form of remembering is accom- the question becomes one of asking where in the natural panied by the autonoetic and chronesthetic consciousness world these two processes might intersect, in which spe- that accompanies human episodic recollections. Indeed, cies, and under what conditions. One classic candidate is we have argued that the ability to remember the ‘what- the food-caching behavior of corvids, members of the where-and-when’ of unique past episodes is the hallmark crow family that include jays, magpies, and ravens as of episodic memory that can be tested in animals.